1. Field of the Invention
This invention relates to an improved method of adding silver to the surface of coral sand which contains calcium carbonate and which is useful for sterilizing water. Coral sand has been attracting attention as a purifying agent for water in recent years.
2. Background of the Art
Coral sand is a porous material having an extremely large specific surface area, but is different from general calcium carbonate.
As a purifying agent for water, it is known to add silver to the surfaces of base agents, such as activated carbon, coral sand, and the like, and to install same into a water purifier for use.
As a method of adding silver to these base agents, a method using the so-called silver mirror reaction is known. Aqueous ammonia is added to a silver nitrate solution to form diamine argenate, coral sand is dipped therein, and then a reducing solution, such as formalin, glucose, and the like, is added to deposit silver on the coral sand (Japanese Patent Publication No. 60-33403).
As another method, it is known to add silver nitrate to coral sand to cover its surface with silver carbonate, to pour a hydrogen peroxide solution thereon to reduce the silver carbonate to silver, and to thereby obtain coral sand having silver adhered on the surface (Japanese Patent Publication No. 63-34228).
However, these prior art methods have the following drawbacks.
In the method employing the silver mirror reaction, a complicated operation is required to provide addition amounts of various reagents and temperature conditions to cause the reduction reaction. Because the reduced silver adheres not only to the coral sand but also to the vessel, impregnation efficiency of silver is poor.
Further, ammonia, which is used as the added reagent, smells bad even if it only remains in small quantities, and it is not suitable for use in a method of adding silver to a purifying agent.
In the case where silver carbonate is reduced by hydrogen peroxide solution to silver after formation of silver carbonate with silver nitrate, there is no bad smell because ammonium is not employed, but the hydrogen peroxide solution is a material used also as a bleaching agent and is known to have a bad effect on animals. Thus, it has some problems when used as an additive (reducing agent) for a purifying agent for drinking water.